This doesn't center the menu, because the theme doesn't have a fixed width: it stretches and shrinks, to adapt to different screen resolutions and devices.
To really center the menu, turn the above to margin: 0; and add the following:

I don't know what you did, but none of my suggestions has been added to your custom CSS. No only that, several of your own previous additions are now missing. (For the most glaring example, where are the bg images to the menu items?)